With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using a magnetic recording technology is continuously increasing at an annual rate of approximately 60%. In recent years, an information recording capacity exceeding 320 gigabytes per platter has been demanded in a perpendicular magnetic recording medium with a 2.5 inch diameter for use in an HDD or the like. To fulfill such a demand, an information recording density exceeding 500 GBit/square inch is desired to be achieved.
Important factors for achieving a high recording density in a perpendicular magnetic disk include securement of electromagnetic conversion characteristics such as improvement in TPI (Tracks per Inch) achieved by narrowing a track width, an SNR (Signal to Noise Ratio) at an improvement time of BPI (Bits per Inch), or an overwrite characteristic (OW characteristic), further, securement of resistance to thermal fluctuation in such a state that a recording bit has become small due to the above-described matter, and the like. Of them, improvement in SNR under a high recording density condition is important.
Since a granular magnetic layer forms fine particles due to separation of an oxide phase and a metal phase from each other, it is effective for achieving a high SNR. Though particles in the granular magnetic layer must be made fine for achieving a high recording density, magnetic energy of particles also becomes small due to the fineness, which results in a problem of the thermal fluctuation. On the other hand, in order to avoid the problem of the thermal fluctuation, it is necessary to enhance magnetic anisotropy in a medium, but since a coercive force becomes too high, writing becomes difficult. That is, the coercive force and the overwrite characteristic are originally in a tradeoff relationship, but there is a demand that both of them be improved.
In Patent Document 1, an auxiliary layer for writing (corresponding to an auxiliary recording layer) formed so as to be brought into contact with a main recording layer above or below the main recording layer has been proposed. In Patent Document 1, the main recording layer is a magnetic layer having a granular structure and the auxiliary layer for writing is composed of CoCr alloy (for example, CoCrPtB). In Patent Document 1, there is a description that both of the overwrite characteristic and the resistance to thermal fluctuation can be achieved by providing the auxiliary layer for writing.